1. Field of the Invention
The present invention relates to a developing device employed for an image forming apparatus, e.g., a laser printing machine and a copying machine in use with an electrophotographic picture.
2. Description of the Related Art
The image forming apparatus such as a laser printing machine and a copying machine in use with an electrophotographic picture basically takes a printing process including of 1) forming an electrostatic latent image on an image carrier (photosensitive body) rotatably supported, 2) supplying developer (toner particles) from a developing device onto the latent image to visualize the latent image as a toner image, and 3) transferring the toner image from the image carrier to a recording medium, e.g., a recording paper, and 4) fixing the toner image on the recording medium One of known developing devices that are applicable to this type of the image forming apparatus uses a developer having two components, toner particles and carrier particles.
The above developing device using the two-component developer will be described below. A two-component developer is mixed and blended in a developer container by stirring. During the mixing/blending process, the toner particles frictionally contact with the carrier particles, and those particles are charged to have a predetermined charge intensity. The thus charged developer is led from the developer container to a developer carrying member (developing roller), which includes a plural number of magnets, and is layered on the peripheral outer surface of the developing roller. With the rotation of the developing roller, the developer that is layered on the outer surface of the developing roller is moved forward under a doctor blade, which is disposed close to the outer surface of the developing roller. The developer is uniformly layered over the outer surface of the developing roller with the aid of the doctor blade when the developer moves under the blade. With a further turn of the developing roller, a uniform layer of developer is moved to a location (developing zone) being confronted with the surface of the latent image carrier. In the developing zone, a latent image formed on the image carrier is developed in a manner that the developer uniformly layered on the developing roller is brought into contact with the image carrier. At this time, a bias potential (referred to as a developing bias) has been applied to the developing roller so as to guide the developer to only an image area of the latent image, of the image area and a nonimage area. Under the developing bias, the developer is attracted to the image area of the latent image to develop or visualize the latent image.
The two-component developing device is devised in many varieties. One of these developing devices is a hybrid developing device. The hybrid developing device includes a plural number (e.g., two) of developing rollers oppositely disposed with respect to the developing zone. The rotating directions of the developing rollers are opposite to each other.
Terms "forward turn developing roller" and "reverse turn developing roller" will be used in the description of the hybrid developing device to follow. The "forward turn roller" means a developing roller which is rotated counterclockwise when the image carrier is rotated clockwise. The peripheral outer surfaces of both the rotary members move in the same direction when observed in the developing zone.
The "reverse turn developing roller" means a developing roller which is rotated in the same direction as of the image carrier. For example, when the image carrier is rotated clockwise, the "reverse turn developing roller" is also rotated clockwise. The moving directions of the peripheral outer surfaces of the rotating members are opposite to each other when observed in the developing zone.
In this type of the developing device, a ratio of a rotating speed of the developing roller to that of the image carrier, i.e., a peripheral speed ratio, is selected to be over 1.
Disadvantages of the conventional hybrid developing device will be described. When a toner image, which is formed through the developing process by only the forward turn developing roller, is observed, the toner layer is thinned and/or no toner layer is formed at the trailing end of the toner image, which is located upstream when viewed in the rotating direction of the image carrier. Therefore, the toner image is blurred at the trailing end. This image defect of the image will be referred to as a "trailing end blurring phenomenon".
On the other hand, the toner layer which is formed through the developing process by only the reverse turn developing roller is thinned and/or no toner layer is formed at the leading end of the toner image. Therefore, the toner image is blurred at the leading end. This image defect will be referred to as a "leading end blurring phenomenon".
Those image defects of the hybrid developing device can be indeed overcome in a manner that the forward turn developing roller supplementarily supplies toner particles to the defective portion, i.e., the toner-layer thinned leading end of the toner image developed by the reverse turn developing roller, while the reverse turn developing roller supplementarily supplies toner particles to the defective portion, i.e., the toner-layer thinned trailing end of the toner image developed by the forward turn developing roller.
However, this approach to overcome the image defects fails to achieve the above object in some setting conditions, however.